1. Field of the Invention
The present invention relates to casting molten metal and, more particularly, to an apparatus, system and method for forming and reworking metallic casts.
2. Prior Art
Titanium and like metals present a great challenge to efficient manipulation and forming, since they can be heated only with special precautions to prevent contamination of the metal from the environment. This is particularly true in making medical and dental castings, and even in forming ingots, because these metals can be safely melted only in inert or highly rarefied atmospheres.
Prior art in the field of forming molten metal, and particularly forming molten titanium and the like, may be very roughly divided into tilt-to-pour, bottom-pour, and countergravity systems. Each has its own drawbacks. Another type of method of forming or casting molten metal is known as spin or centrifugal casting. Ohara Company, Ltd. of Osaka, Japan manufactures and sells a casting machine called "Titaniumer" that spins a mold to allow molten metal from a ceramic crucible to flow thereinto. U.S. Pat. No. 4,700,769 to Ohara et al. and U.S. Pat. No. 4,280,551 to Ohara also disclose casting apparatus for titanium and titanium alloys.
Use of titanium and its alloys as medical and dental implants is well recognized Such recognition has been attained due to their excellent corrosion resistance and biocompatibility in the body environment. The ability to passivate (to form an invisible and tightly adherent inert oxide film which is quickly reestablished whenever it is subjected to abrasion) instantaneously in air at room temperature imparts these excellent chemical and biological qualities to pure titanium and its alloys. Their corrosion resistance is comparable to or exceeding that of cobalt chrome alloys or stainless steel.
Low specific gravity and excellent mechanical properties of titanium and its alloys also make them ideal restorative materials for dentistry. The weight of titanium, per unit volume, is half that of Ni and Co-Cr alloys and one quarter that of the gold alloys. Therefore, for a given volume of the prosthesis, only one half and one-quarter of the weight of titanium is required. Low thermal conductivity of these materials would also impart no or significantly less hot or cold feelings to the surrounding tissue upon intake of hot and cold fluids. The difficulty of casting titanium and its alloys arise from the fact that they are very high melting, such as greater than 1,700.degree. C., and their strong affinity for oxygen, nitrogen, hydrogen and carbon in the molten condition. Attempts to circumvent these problems have focussed on the casting environment, specific gravity consideration and the mold materials. To force the molten metal into the intricate mold cavities two methods have been used. A first method does not use a vacuum, but rather, uses argon gas saturation in combination with centrifugal force. However, this first type of method does not provide for the removal of all reactive gases or elements, such as oxygen, nitrogen, hydrogen, etc. Thus, these reactive elements contaminate the titanium during its cast. In addition, the ceramic crucible used for melting is another source of contamination. A second method uses pressure differential force, argon saturation, and mediocre vacuum. However, this second type of method does not provide for the fast casting of molten material into a mold. This method also tends to aspirate the metal thereby creating forthing. Because of the fact that titanium and its alloys have very high melting points (greater than 1,700.degree. C.) and low density, if not cast into the mold very fast, portions of the molten material can cool and solidify before the mold is completely filled, thus resulting in an improper cast. Attempts to improve the castability by using hot molds in the past, only further contaminated the metals and alloys.